The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include gerotor displacement mechanisms utilizing low-speed, commutating valving.
In a conventional gerotor motor utilizing low-speed, commutating valve (i.e., the rotary valve element rotates at the speed of rotation of the gerotor star rather than at the orbiting speed of the star) the valving action has been accomplished by means of a rotary valve member and a stationary valve member, with both valve members being separate from the gerotor displacement mechanism.
In recent years, those skilled in the art have developed what may be termed a "valve-in-star" (VIS) gerotor motor, an example of which is illustrated and described in U.S. Pat. No. 4,741,681, assigned to the assignee of the present invention and incorporated herein by reference. In a VIS motor, the commutating valving action is accomplished at an interface between the orbiting and rotating gerotor star, and an adjacent, stationary valve plate, which is typically part of the motor housing.
Although "commutating" valving action is well known to those skilled in the gerotor motor art, a brief explanation will be provided herein. In a typical gerotor motor, the ring member defines a plurality N+1 of internal teeth, and the orbiting and rotating star defines a plurality N of external teeth. The stationary valve member then defines a plurality N+1 valve passages communicating with the expanding and contracting fluid volume chambers of the gerotor, while the rotary valve member (orbiting and rotating star in the case of a VIS motor) defines a plurality N of fluid ports at high pressure ("system pressure"), and a plurality N of fluid ports at low pressure (return or exhaust). The progressive fluid communication between each of the N ports and each of the N+1 fluid passages, as the star orbits and rotates, comprises the commutating valving.
In a typical VIS motor, system pressure is communicated through the end cap, and the stationary valve surface, axially to a transverse face of the gerotor star, thus subjecting the star to a substantial axial separating force, tending to bias the star away from the stationary valve surface. Therefore, it has been necessary to provide a means to accomplish "overbalance" of the star, such that there is a net force tending to bias the star toward the stationary valve surface. This may be accomplished by providing the "backside" of the star (i.e., the side of the star opposite the end cap) with a pressure overbalance region, and then communicating system pressure into the region, from whichever set of star ports contains high pressure. Such an arrangement is illustrated and described in U.S. Pat. No. 4,976,594, assigned to the assignee of the present invention and incorporated herein by reference.
In commercial VIS motors produced by the assignee of the present invention (the Hydraulics Operations Worldwide of Eaton Corporation), communication of fluid to and from the star is accomplished by means of a pair of pressure chambers (or regions) defined by the end cap assembly. The first pressure chamber is annular, and the second pressure chamber is circular and is surrounded by the first pressure chamber. The above-described pressure chamber arrangement is illustrated and described in greater detail in both of the above-incorporated patents. Although the operating performance of the pressure chamber arrangement described above has been generally satisfactory, it has made pressure balancing of the star quite difficult. As will be understood by those skilled in the art of VIS motors, the annular, first pressure chamber has a larger area than the circular, second pressure chamber. As a result, when the second pressure chamber contains high pressure (for example, when the motor is operating counter-clockwise (CCW)), there is a much smaller hydraulic separating force acting on the star than when the first pressure chamber contains high pressure (when the motor is operating clockwise (CW)). Therefore, for a given pressure balance area on the backside of the star, there will be a much greater overbalance on the star when the motor is operating CCW than when the motor is operating CW.
As an example, during the development of the motor comprising the subject embodiment, using the pressure chamber arrangement described above, there was a 24% overbalance in the CCW direction, but a 0% "overbalance" in the CW direction. Those skilled in the art will recognize that a 0% overbalance is, in reality, no overbalance at all, and there is a great potential for axial separation of the star from the stationary valve plate, followed by cross-port leakage and stalling of the motor.
A seemingly obvious solution to the above problem would be to reduce the area of the annular, first pressure chamber, i.e., reduce the radial dimension of the first pressure chamber. However, reducing the area of the first pressure chamber, which must communicate with ports defined by an orbiting and rotating star, would typically reduce the area of communication therebetween enough to increase the pressure differential (pressure drop) across the motor to an undesirably high level.